The canals are not idle they are in use constantly by the leisure industry, which with canal enthusiasts have restored and now maintain most of the system.
The government would only have to encourage their use. Private industry is already there see here.
Yeah, I remember that post. What I meant was that they're not generally being used for what, logically, should be their main purpose: heavy, non-time critical transport. As you've pointed out, modern-build narrowboats can hold 100 tons: the same amount of goods as two or three HGVs.
As you say, its not that the canals - and rivers - aren't in use at all. I did live near the Thames until recently, you know - Staines was only a short journey from Egham. Plenty of narrowboats along there...